Wire Fill

Free Wire Fill calculator with clear step-by-step results.

About Wire Fill

Fill is total cable cross-sectional area over conduit internal area. The limit exists so cables can be drawn in without stripping insulation and so heat can dissipate, and the bend count matters independently - most codes cap a run at 360 degrees of bend between draw points, which is four right angles.

Formulas

Fill percentage

Fill % = cables x pi x (cable OD / 2)^2 / (pi x (conduit ID / 2)^2) x 100

Maximum cables

Max = floor(conduit area x target fill / cable area)

Worked example

With the default values below, Wire Fill returns Conduit fill of 40.56%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Conduit internal diameteraccepts 5–200 mm: 25 mm
Cable outer diameteraccepts 0.5–80 mm: 6.5 mm
Number of cablesaccepts 1–100: 6
Maximum fill allowedaccepts 10–60 %: 40 %
Bends in the runaccepts 0–12: 3

Results

Conduit fill: 40.56% %
Maximum cables at the target fill: 5 cables
Spare capacity: 0 cables
Bend assessment: Three bends - at the usual limit

Step by step

1Values used
2Fill percentage
3Maximum cables
4Conduit fill= 40.56 %
5Maximum cables at the target fill= 5 cables
6Spare capacity= 0 cables
7Bend assessment= Three bends - at the usual limit

Assumptions and limits

The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Conduit fill, bend radius, draw-point spacing and conductor derating are governed by electrical codes that differ by jurisdiction. Installations must be designed and inspected by a licensed electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Conduit fill, bend radius, draw-point spacing and conductor derating are governed by electrical codes that differ by jurisdiction. Installations must be designed and inspected by a licensed electrician.

Frequently Asked Questions

Why is 40% the usual limit?

It is the code allowance for three or more conductors and balances pulling tension against thermal dissipation. One conductor is allowed 53% and two only 31%.

Does fill affect current rating?

Yes, separately. More conductors bundled together means mutual heating, which triggers ampacity derating on top of the geometric fill limit.